A quick check of the Oxford English Dictionary reveals that the earliest mention of the term 'Middle Ages' or 'Middle Age' as it was in this text was in 1570's J. Foxe Actes & Monumentes (rev. ed.) I.III 204/1 "The primitive tyme of the church....the middle age, and ..these our latter dayes of the church"
William Camden, who wrote the great Britannia, used the term in 1605 writing, "I will onely giue you a taste of some midle age, which was so ouercast with darke clouds, or rather thicke fogges of ignorance."
H. Wotton wrote in 1624 "after the reuiuing and repolishing of good Literature, (which the combustions and tumults of the middle Age had vnciuillized)"
Edward Gibbon also used the term, "During the middle ages, (from the ninth to the twelfth century) whilst Christianity was advancing with a slow progress into the North." 1776
Henry Hallam considered the Middle Ages to run from the invasion of France by Clovis in 490 A.D. to the invasion of Naples by Charles VIII in 1494.
I consider it to start with the fall of Rome in 410 A.D. and end with the fall of Constantinople in 1453. It seems neater that way. Begins with the fall of the western half of the Roman empire and ends with the fall of the eastern half.
The OED goes on to say that the term comes from the Latin media aetas, medium aevum, medium tempus and was especially used in Basle. ( at the university I assume) 1469 in Rome was the first use of media tempestas but they didn't list who the author/ authors were. Perhaps it evolved from Petrach since the concept of the "Dark Ages" is credited to him as well. Historians in the Middle Ages went with the 'Six Ages of the World' of Augustine and considered themselves in the last age before the Apocalypse. People have been predicting the Rapture forever.